Introduction

Decreased quantities of total-column ozone have been observed over large parts of the globe, permitting an increased penetration of solar ultraviolet-B (UV-B, 280 nm - 315 nm) to the earth's surface (UNEP, 1998). The first reports of potential stratospheric ozone reduction were made more than 30 years ago (Johnston, 1971; Crutzen, 1972). The UV-B radiation on the earth's surface has increased by 6% -14% since the early 1980s (UNEP, 2002). More than 600 papers have been published with much attention directed on the effects of UV-B radiation on higher plants (Caldwell et al., 1998). Studies of the effects of increased solar UV-B at the ecosystem level (Caldwell et al., 1998) have only been undertaken in the past

10 years. Approximately 400 species of plants and cultivars have been screened for sensitivity to UV-B radiation, and of these, about two-thirds were found to be sensitive in some parameter (Sullivan and Rozema, 1999; Sullivan et al., 2003). The balance between damage and protection varies among species, even within varieties of crop species. Many species and varieties can accommodate increased UV-B. Tolerance of elevated UV-B by some species and crop varieties provides opportunities for genetic engineering and breeding to deal with potential crop yield reductions due to elevated UV-B in agricultural systems (UNEP, 1998).

Research indicates that increased UV-B exerts effects more often through altered patterns of gene activity than through damage. These UV-B effects on regulation manifest themselves in many ways, including changes in plant form and production of plant chemicals not directly involved in primary metabolism (UNEP, 1998). Much evidence suggests that plants have evolved two major strategies for resistance to UV-B radiation that involve repair and avoidance mechanisms. The former includes repair of DNA damages by excision repair or by repair of pyrimidine-dimers as photolyase, activated by UV-A and photosynthetically active radiation (PAR) (Taylor et al., 1997). The latter primarily includes epidermal screening of UV-B radiation to protect the mesophyll tissue of a leaf by the accumulation of UV-absorbing compounds in cell vacuoles and/or cell walls of the epidermis (Caldwell et al., 1983; Hutzler et al., 1998). Research has shown that flavonoids and related phenolics play an important role in plant defense against the UV-B radiation (Caldwell et al., 1983; Tevini et al., 1991; Day et al., 1992; Day, 1993; Li et al., 1993; Beggs and Wellmann, 1994; Day et al., 1994; Karabourniotis and Fasseas, 1996; Reuber et al., 1996; Bornman et al., 1997; Hutzler et al., 1998; Karabourniotis et al., 1998; Laakso et al., 2000; Sullivan et al., 2005).

Structural and biochemical changes induced by enhanced levels of UV-B radiation ultimately modify the penetration of UV radiation into plants. In order for UV radiation to be effective in plants, it must effectively penetrate into the tissues and be absorbed. The ability to predict the consequences of enhanced ambient UV-B levels on plants depends in part on our understanding of how much of this radiation reaches the chromophores within the mesophyll (Day, 1993). Ultraviolet penetration varies with plant species. Penetration of UV-B was found to be the greatest in herbaceous dicotyledons (broad-leafed plants) and was progressively less in woody dicotyledons, grasses and conifers (Day et al., 1992). The UV penetration also changes with leaf age; younger leaves attenuate UV-B radiation less than do the more mature leaves in some conifers (DeLucia et al., 1991; DeLucia et al., 1992).

Although some 400 plant species and cultivars have been studied, the vast majority tested have been herbaceous, annual agricultural species grown in laboratory or glasshouse conditions. Fewer than 5% of the studies have been conducted under field conditions. Relatively little information exists on the effects of UV-B radiation on forest tree species (Caldwell et al., 1998), which account for more than 80% of global net primary production (Whittaker, 1975; Barnes et al., 1998). Tropical forests have received very little attention with respect to the ozone reduction problem (Searles et al., 1995; Caldwell et al., 1998) even though they represent nearly one half of global productivity and much of the total tree species diversity. Our knowledge is far from complete in regard to the effects of enhanced UV-B on trees and forest communities in various landscapes. The diverse range of UV-B radiation responses observed within annual plant species suggests that direct extrapolations from these species to long-lived woody plants many not be feasible. Therefore, it would be useful to know the differences in UV-B screening effectiveness in various tree species and to determine what general leaf properties are responsible for these differences. This paper is a result of a USDA-funded collaborative research project involving the scientists from Southern University, the USDA-FS Northeast Research Station, the USDA UV-B Monitoring and Research Program (UVMRP), and the University of Vermont. The research was the first in the southern US that focused on assessing UV-B radiation tolerance characteristics in diverse southern broadleaf tree species (Table 18.1, column 1). The specific objectives were to: (1) measure leaf reflectance, transmittance, and absorption of UV (280 nm - 400 nm) and visible (400 nm - 760 nm) radiation spectrums on a whole-leaf basis, (2) measure the light distribution and depth of UV-B light penetration (310 nm) into the leaves, (3) investigate leaf surface morphology and leaf anatomy, and (4) measure the total concentration of UV-B-absorbing compounds in the leaves throughout the growing season.

Table 18.1 Leaf reflectance (R^), transmittance T), and absorbance (A^to 300 nm

UV-B radiation on a whole leaf basis, measured from the mature leaves of 35 southern tree species, grown in Baton Rouge, Louisiana, USA

Scientific name-common name

Broadleaf evergreen trees

Magnolia grandiflora - Southern magnolia

Magnolia virginiana - Sweet bay magnolia

Quercus glauca - Blue Japanese evergreen oak

Quercus virginiana - Southern live oak

Deciduous trees

Acer rubrum - Red maple

Betula nigra - River birch

Carya cordiformis - Bitternut hickory

Carya illinoinensis - Pecan

Carya tomentosa - Mockernut hickory

Castanea dentata - American chestnut

Celis laevigata - Sugar hackberry

Whole leaf Whole leaf Whole leaf reflectance to transmittance absorbance to 300nm (%) to 300nm (%) 300nm (%)

Rl=300nm Tl=300nm Al=300nm

6.051 0.079 93.870

6.004 0.003 93.993

6.756 0.005 93.239

8.475 0.000 91.525

6.017 0.003 93.979

6.866 0.004 93.130

6.044 0.001 93.957

8.576 0.010 91.415

6.223 0.002 93.775

7.852 0.000 92.148

5.388 0.005 94.607

(Continued)

Whole leaf

Whole leaf

Whole leaf

Scientific name-common name

reflectance to

transmittance

absorbance to

300nm (%)

to 300nm (%)

300nm (%)

R=300nm

Tl=300nm

Al=300nm

Cercis canadensis - Red bud

4.766

0.000

95.234

Cornus florida - Dogwood

8.610

0.048

91.342

Fagus grandifolia - American beech

6.970

0.003

93.027

Fraxinus pennsylvanica - Green ash

5.467

0.003

94.530

Fraxinus velutina - Arizona ash

6.570

0.003

93.427

Liquidamber styraciflua - Sweetgum

7.464

0.001

92.536

Liriodendron tulipifera - Yellow poplar

4.807

0.002

95.191

Morus rubra - Red mulberry

7.440

0.023

92.537

Platanus occidentails - Sycamore

6.824

0.012

93.164

Populus deltoides - Cottonwood

7.337

0.001

92.662

Pyrus calletyana "Bradford" - Bradford pear

6.088

0.002

93.910

Quercus acutissima - Sawtooth oak

6.634

0.167

93.199

Quercus alba - White oak

7.073

0.005

92.922

Quercus michauxii - Swamp chestnut oak

5.637

0.004

94.359

Quercus nigra - Water oak

7.186

0.001

92.813

Quercus palustris - Pin oak

5.843

0.005

94.152

Quercus phellos - Willow oak

5.503

0.008

94.490

Quercus shumardii- Shumard oak

8.144

0.005

91.851

Quercus stellata - Post oak

6.922

0.001

93.077

Quercus falcata - Southern red oak

5.974

0.004

94.022

Quercus falcata var. pagodifolia - Cherrybark oak

5.700

0.008

94.291

Sapium sebufrum - Chinese tallow

5.998

0.002

94.000

Ulmus americans - American elm

5.758

0.002

94.240

Ulmus parvifolia - Chinese elm

6.387

0.004

93.608

Mean

6.553

0.012

93.435

Max

8.610

0.167

95.234

Min

4.766

0.000

91.342

SD

1.005

0.031

1.008

The project has established a database of leaf optical properties (reflectance, transmittance, and absorbance to UV-B, UV-A, and visible light), leaf UV-B absorbing compound profiles, depth of UV-B penetration into leaves, and leaf anatomical characteristics. It is expected that through the comparative analyses of these properties across the species, we may have a better understanding of the biophysical and biochemical aspects of UV-B tolerance characteristics in diverse broadleaf trees in the South.

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